Inorg. Chem. 1983, 22, 1550-1557
1550
Contribution from the Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138, and the Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39
Analogues of the [4Fe-4S]+ Sites of Reduced Ferredoxins: Structural and Spectroscopic -C6H4Br)4]in Crystalline and Solution Phases Properties of [(CzH5)4N]3[Fe4S4(S-p D. W. STEPHAN,la*bG. C. PAPAEFTHYMIOU,lC R. B. FRANKEL,lCand R. H. HOLM*lb
Received August 17, 1982
'-,
The structural properties of the reduced clusters [ Fe,S,(SR),] synthetic analogues of the [4Fe-4S]+ sites in ferredoxin proteins, have been augmented by preparation and single-crystal X-ray analysis of the title compound. (Et4N)'[Fe4S4( S p C 6 H 4 B r ) , ] (1) crystallizes in orthorhombic space group Fdd2 with n = 36.218 (10) A, b = 30.972 (7) A, c = 11.631 (3) A, and Z = 8. The cluster has an imposed twofold symmetry axis and exhibits a distortion from idealized Td symmetry of the [4Fe-4S]+ core that is different from those of (Et3MeN)3[Fe4S4(SPh)4](2) and (Et,N),[Fe4S4(SCH2Ph),] (3), the only other structurally defined reduced clusters. One description of the cubane-type core involves four short and eight long Fe-S bonds with mean values of 2.292 (2) and 2.313 (8) A, respectively. Solid 1 exhibits an especially well-resolved Mossbauer spectrum a t 4.2 K consisting of two doublets of 1: 1 intensity having isomer shifts consistent with the mean oxidation state and a quadrupole splitting difference of 1.13 mm/s. This spectrum is distinguishable from those of 2 and 3. Internal hyperfine fields a t the two resolvable Fe sites have been estimated from an analysis of magnetically perturbed spectra. The solution spectrum of 1 at 4.2-77 K is the same as those of all other [Fe4S,(SR),l3- clusters examined previously. These closely resemble that of 2, which has a tetragonally elongated core structure, indicating a change to, or nearly to, this configuration upon passing from the solid to the solution state. This conclusion is also reached by comparison of EPR spectra in the two states. Compound 1 is the third containing a structurally characterized reduced cluster and sixth overall to exhibit, by magnetic and/or spectroscopic property/structure correlations, this structural change. The results of this investigation further substantiate the unperturbed structural change
-
-
[Fe4S4(SR)4]2-(compressed DU)
+e-
[Fe4S4(SR)4]3-(elongated DM)
attendant to electron transfer. This redox couple is isoelectronic with the prevalent electron-transfer process of ferredoxins containing [4Fe-4S] sites.
Introduction The prevalent electron-transfer process of ferredoxin (Fd) proteins containing cubane-type [4Fe-4S] redox sites2 is that represented by the couple (1). The corresponding isoelectronic
+ e- + Fdrd( [4Fe-4SIf) [Fe4S4(SR)4]2-+ e- + [Fe4S4(SR),13-
Fd,,([4Fe-4S]2+)
(1) (2)
couple of synthetic analogues of these sites is (2). Isolation of examples of both oxidized3p4and reducedks members of the latter couple has permitted investigation of several important features of cluster electron transfer. Chief among these are the fast electron self-exchange rates (k 106-107M-' s-l at -298 K9)and the structural change attendant to electron t r a n ~ f e r .The ~ ~ ~latter is depicted schematically in Figure 1, where it is seen that the principal alteration is compression/elongation of the [4Fe-4S] core along the idealized 4 axis. Evidence for the structural change in Figure 1 has been derived7q8from the following observations. (i) Crystal struc-
-
(1) (a) National Science and Engineering Research Council of Canada/ NATO Postdoctoral Fellow, 1980-1982. (b) Harvard University. (c) MIT.
(2) Sweeney, W. V.; Rabinowitz, J. C. Annu. Reu. Biochem. 1980,49, 139. (3) Holm, R. H. Acc. Chem. Res. 1977, 10, 427. Holm, R. H.; Ibers, J. A. In "Iron-Sulfur Proteins"; Lovenberg, W., Ed.; Academic Press: New York, 1977; Vol. 111, Chapter 7. (4) Berg, J. M.; Holm, R. H. In "Metal Ions in Biology"; Spiro, T. G., Ed.; Wiley: New York, 1972; Vol. 4, Chapter 1. (5) Cambray, J.; Lane, R. W.; Wedd, A. G.; Johnson, R. W.; Holm, R. H. Inorg. Chem. 1977, 16, 2565. (6) Reynolds, J. G.; Laskowski, E. J.; Holm, R. H. J. Am. Chem. SOC.1978, 100, 5315. (7) Laskowski, E. J.; Frankel, R. B.; Gillum, W. 0.;Papaefthymiou, G. C.; Renaud, J.; Ibers, J. A,; Holm, R. H. J. A m . Chem. SOC.1978, 100, 5322. (8) Laskowski, E. J.; Reynolds, J. G.; Frankel, R. B.; Foner, S.; Papaefthymiou, G. C.; Holm, R. H. J. Am. Chem. SOC.1979, 101, 6562. (9) Reynolds, J. G.; Coyle, C. L.; Holm, R H. J. Am. Chem. SOC.1980, 102, 4350. 0020-1669/83/1322-1550$01.50/0
tures of some seven compounds containing clusters with the [4Fe-4SI2+ core oxidation l e ~ e 1 ~reveal J ~ ' ~the dominant deviation from idealized core T d symmetry to have the form of a compressed tetragonal distortion. This distortion, while variable in extent, persists in quite different crystalline environmentsI2 and in clusters whose terminal ligands are wholly1&l3or partially14 thiolates or are other groups entirely.I5J6 Hence, the compressed DZdstructure appears to be the intrinsically stable form of the [4Fe-4SI2+ core.I7 (ii) Distortions of the [4Fe-4S]+ cores in crystalline (Et3MeN)3[Fe,S4(SPh),l7 and (Et4N),[Fe4S4(SCH2Ph),]l 8 are different from each other (Figure l), and their solid-state magnetic and spectroscopic properties are d i ~ t i n c t . ~With use of these structure/property correlations, nine compounds of seven clusters [Fe,S,(SR),] 3- have been broadly classified as containing elongated tetragonal or nontetragonal core structures in the solid state.s (iii) Regardless of their classification, spectroscopic and magnetic properties of all clusters in frozen solutions are essentially the same and are in agreement with those of crystalline (Et3MeN)3[Fe4S4(SPh)4J, in which the (Et4N)2[Fe4S4(SCH2Ph)4]: Averill, B. A,; Herskovitz, T.; Holm, R. H.; Ibers, J. A. J. Am. Chem. SOC.1973, 95, 3523. (Me4N)2[Fe4S4(SPh)4]: Que, L., Jr.; Bobrik, M. A,; Ibers, J. A,; Holm, R. H. J. Am. Chem. SOC.1974, 96, 4168. Na5(n-Bu4N)[Fe4S4(SCH2CH2C02)4].5C5H9NO: Carrell, H. L.; Glusker, J. P.; Job, R.; Bruice, T. C. J. Am. Chem. SOC.1977,99,3683. (Me4N)2[Fe4S4(SCH2CH20H)4]: Christou, G.; Garner, C. D.; Drew, M. G. B.; Cammack, R. J. Chem. SOC.,Dalton Trans. 1981, 1550. (Ph4P),[Fe4S4(SPh)2C12]: Coucouvanis, D.; Kanatzidis, M.; Simhon, E.; Baenziger, N. C. J. Am. Chem. SOC.1982, 104, 1874. (Et4N)2[Fe4S4C14]:Bobrik, M. A.; Hodgson, K. 0.;Holm, R. H. Inorg. Chem. 1977, 16, 1851. (Et4N)2[Fe4S4(OPh)4]:Cleland, W. E.; Averill, B. A. Inorg. Chim. Acra 1981, 56, L9. Averill, B. A,; Ibers, J. A,, unpublished results. Recent resonance Raman spectroscopic results for (n-Bu4N),[Fe4S4(SCH2Ph)4]suggest a different type of distortion of the cluster (Spiro, T. G., private communication). The crystal structure of this compound is not available. Berg, J. M.; Hodgson, K. 0.;Holm, R. H. J . A m . Chem. SOC.1979, 101, 4586.
0 1983 American Chemical Society
Inorganic Chemistry, Vol. 22, No. 10, 1983 1551
Analogues of [4Fe-4S]+ Sites of Reduced Ferredoxins
3-
-4
2
2-
4
4'
2.2dl5
3-
'SPh
2
3-
3-
[Fe4S4(S-p-C6y4Br)J
'-
[Fa4 S4 ECHOphI4]
Figure 1. Schematic structures of [Fe4S4(SR),13- clusters in the crystalline state and a simplified representation of the structural conversion of the Fe4S4core in [Fe4S4(SR)4]2-s3-electron transfer in solution. Idealized or actual core symmetry axes and mean values of Fe-SR and sets of longer (dark lines) and shorter Fe-S bonds are shown.
cluster has an elongated DZdconfiguration. If it is accepted that a frozen solution is less likely to provide perturbing influences than is a crystalline phase, the elongated Dzd structure appears to be the intrinsically stable form of the [4Fe-4S]+ core. The properties of the analogue couple (2) are relevant to the biological couple (1) in that they are not influenced by protein structure and environment. As in other aspects of the synthetic analogue approach to metalloprotein site struct u r e ~ , ~analogue ,'~ properties are intrinsic and provide a base line against which protein influences can be assessed. With the clusters [Fe4S4(SR),13- in the solid state, however, the foregoing evidence shows that their properties are influenced by the environment, perhaps accidentally simulating to an extent protein effects on [4Fe-4S]+ cores. This raises the questions of the variability of core distortions, sensitivity of electronic properties to structure, and the generality of formation of the elongated DZdsolution structure from nontetragonal solid-state structures. Examination of these matters has been impeded by the considerable difficulty in obtaining single crystals of reduced cluster salts suitable for accurate X-ray structural determinations. The compound (Et4N),[Fe4S4(S-p-C6H4Br)4]has recently been obtained in a suitable crystalline form. Its structure is reported here together with certain spectroscopic information bearing on the preceding points. Experimental Section Preparation of Compounds. All manipulations were performed under a pure argon or dinitrogen atmosphere with use of degassed solvents. (Et4N)z[Fe4S,(S-p-C6H4Br)4]. This compound was prepared by direct tetramer synthesis using elemental sulfur as the source of sulfide.20*z1 Sodium (1.52 g, 66 mmol) was dissolved in 200 mL of methanol. To this solution was added 12.5 g (66 mmol) of p bromobenzenethiol (Aldrich Chemical Co.) followed by 2.7 g (17 mmol) of ferric chloride dissolved in 30 m L of methanol and 0.53 g (17 mmol) of sulfur. The reaction mixture was stirred overnight and filtered into a solution of 2.1 g (10 mmol) of Et4NBr in 50 mL of methanol. After collection by filtration the compound was cry(19) Ibers, J. A.; Holm, R. H. Science 1980, 209, 223. (20) Christou, G.; Gamer, C. D. J. Chem. SOC.,Dalron Trans. 1979, 1093. (21) Hagen, K. S.;Reynolds, J. G.; Holm, R. H. J . Am. Chem. SOC.1981, 103, 4054.
tallized once from acetonitrile/methanol, affording 2.1 g (41%) of pure product as black crystals. Anal. Calcd for C&56Br4Fe4NzSs: C, 35.21; H, 4.14; Br, 23.43; Fe, 16.37; N , 2.05; S, 18.80. Found: C,35.07;H,4.10;Br,23.26;Fe,16.37;N,2.01;S,18.11. IHNMR (CD3CN): 6 8.31 (m-H), 5.85 (o-H), 3.15 ( N C H J , 1.20 ( N C H z CH,). Redox potentials (cyclic voltammetry, 200 mV/s, D M F solution, vs. SCE): E(2-/3-) = -0.93 V; E(3-/4-) = -1.64 V. (E4N),[Fe4S4(S-p-C6H4Br),]. This compound was prepared by reduction of the preceding compound with sodium acena~hthylenide.~ A 0.2 M solution of the reductant was prepared from 0.15 g of sodium and 1.00 g of sublimed acenaphthylene in 33 mL of hexamethylphosphoramide (HMPA). To a solution of 2.3 g (1.7 mmol) of (Et4N)2[Fe4S4(S-p-C6H4Br)4] and 0.35 g (1.7 mmol) of Et,NBr in 30 mL of H M P A was added dropwise 10 mL of the sodium acenaphthylenide solution. After the reaction mixture was stirred overnight, 60 mL of T H F was added. Cooling to -20 OC caused separation of black crystals. After the product (2.4 g, 96%) was collected by filtration, it was recrystallized on small scales from acetonitrile or acetonitrile/diethyl ether. Anal. Calcd for C4sH,6Br4Fe4N3Ss:C, 38.57; H, 5.13; Br, 21.38; Fe, 14.95; N , 2.81; S, 17.16. Found: C, 38.32; H , 4.87; Br, 21.22; Fe, 15.06; N , 2.65; S, 17.24. IH N M R (CD3CN): 6 10.50 (m-H), 3.15 (NCH2), 1.29 (NCH2CH3). This compound is extremely sensitive to oxidation, especially in solution. X-ray Data Collection and Reduction. Slow crystallization performed by diffusion of diethyl ether into a dilute acetonitrile solution yielded long black needles of (Et4N), [Fe4S4(S-pC6H4Br),]. Suitable single crystals, obtained by cutting the needles, were mounted in glass capillaries under an argon atmosphere. Diffraction experiments were performed on a Nicolet R 3 M diffractometer with graphite-monochromatized Mo Ka radiation. The initial orientation matrix was obtained from 14 machine-centered reflections selected from a rotation photograph. Determination of an orthorhombic crystal system was made from 21 carefully centered high-angle reflections (20' < 28 < 25'). Partial rotation photographs around each axis confirmed mmm or mm2 symmetry. The observed systematic absences from a second partial data set (3O < 28 < 25') were consistent only with space group Fdd2, which was confirmed by solution and refinement of the structure. The 21 centered reflections were used to obtain the final lattice parameters and the orientation matrix. Machine parameters, crystal data, and data collection parameters are summarized in Table I. The crystal selected for data collection gave w scans for several low-angle reflections with full widths at half-heights of 2o(FoZ)) no. of variables goodness of fit R , 41c R,. %
(
S
eBr),]
2
C,,H,,Br,Fe,N,S, (1494.66) 36.218 (10) 30.972 (7) 11.631 (3) orthorhombic Fdd2 8 13 046 (6) 1.52 0.58 X 0.16 X 0.18 {110), (0111 Mo KZ (A 0.710 69 A ) 35.78 51.54 min, 60.63 max 2.0-29.3 (0/20 scan) 0.7 below Ka, to 1.0 above Ka, 0.25 20 of 3-30', +h,+k,il; 20 of 30-45". + / I , +k, +I 1789
formula (mol wt j a, A
)3 [Fe4S,
Figure 2. Structure of the Fe& portion of [Fe4S4(S-~-C6H4Br)4]3as its Et4Nt salt, showing 50% probability ellipsoids, selected interatomic distances, and the atom-labeling scheme. Primed and unprimed atoms are related by the crystallographic twofold symmetry axis.
238 1.456 6.39 6.11
motion of the C atoms led to an isotropic description of these atoms in subsequent refinements. At this point the polarity of the data was inverted. A difference Fourier calculation showed R = 6.59%, indicating that the second polarity is the correct choice. Subsequent refinements were performed by using this polarity of the data. Hydrogen atoms were calculated by assuming trigonal (phenyl rings) and tetrahedral (cations) geometries and a C-H distance of 0.96 A. Temperature factors were set at 1.2 times the isotropic temperature factor of the bonded C atom. Following refinement cycles in which hydrogen contributions were included, H atom positions were recalculated. In the final block-cascade refinement including hydrogen atom contributions, the phenyl rings were constrained to rigid hexagons of anisotropic atoms. The mean shift of the parameters in the last cycle of refinement was 0.003~.A final difference Fourier map showed no peaks of chemical significance. Final R factors are included in Table I. The following results for (Et4N), [Fe4S4(S-p-C6H4Br)4]are tabulated: positional parameters (Table II), interatomic distances and angles of the anion (Table HI), temperature factors (Table S I ) , hydrogen atom coordinates and temperatures factors (Table S-II), best-weighted least-squares planes of the Fe4S4core of the anion (Table S-III), values of 10IFol and 10IFcl (Table S-IV**).
SHELX-76program packages (Nicolet XRD Corp., Fremont, CA). An analytical absorption correction was applied to the complete data set. A total of 1789 reflections with Fo2> 24F:) including Friedel pairs was collected. Structure Solution and Refinement. The positions of the two Fe atoms in the asymmetric unit were determined from a Patterson map. Bromine and sulfur (sulfido) atoms were located from subsequent difference Fourier maps. This partial solution was in agreement with that obtained by direct methods. The remaining non-hydrogen atoms of the cations and anion were found from addition difference Fourier maps. A block-diagonal refinement employing anisotropic descriptions of Fe, S, and Br atoms and isotropic C and N atoms gave R = 8.9%. An analytical absorption correction was then applied. Several more cycles of refinement, in which the phenyl rings were constrained to be regular hexagons of isotropic description and the remaining atoms were treated anisotropically, gave R = 7.5%. Repeated attempts to refine anisotropically the C atoms of the full cation (N(2)C(17-24)) in the asymmetric unit failed. The apparent high degree of thermal Table 11. Positional Parameters (X104) for (Et,N), (Fe,S,(S-p-C,H,Br),] atom Fe(l) Fe(2) S(1) S(2) BI(~)~ Br(2) S(3) S(4) N(1) Ni2) C(l)b,C (22) C(3) C(4) Ci5) C(6)
C(7) a
X
Y
Z
133 ( l ) a 360 (1) 184 (1) 466 (1) 1001 (1) -209 (1) 372 (2) 870 (2) 0 1213 (4) 195 (4) 53 (4) -61 (4) -33 (4) 108 (4) 222 (4) 897 (6)
-415 (1) 171 (1) -542 (1) 210 (2) 2165 (1) -2851 (1) -940 (2) 308 (2) 5000 1371 (6) -1462 (4) -1556 (4) -1974 (4) -2300 (4) -2207 (4) -1788 (4) 838 (5)
0 -1641 (3) -1936 (4) 314 (4) -5101 (5) -85 (4) 1200 (4) -2736 (8) 9266 (23) 784 (20) 806 (16) -281 (16) -544 (16) 280 (16) 1367 (16) 1630 (16) -3292 (13)
C(8)
C(9) C(10) C(11) C(12) (313) C(14) C(15)b (316) (317) C(18) (719) C(20) C(21) C(27.1 C(23) ~(24)
Estimated standard deviations in parentheses in this and succeeding tables. 6.8;
X
Y
Z
1216 (6) 1250 (6) 966 (6) 648 (6) 613 (6) 2475 (9) 2455 (10) 2809 (11) 3204 (1 1) 1292 (6) 986 (9) 847 (8) 953 (15) 1539 (7) 1549 (9) 1051 (12) 1401 (11)
939 (5) 1343 (5) 1645 (5) 1544 (5) 1141 (5) 2084 (8) 1657 (7) 2452 (8) 2458 (13) 1824 (7) 2117 (10) 1217 (8) 1117 (16) 1129 (7) 667 (1 1) 1432 (13) 1551 (14)
-3911 (13) -4434 (13) -4338 (13) -3719 (13) -3196 (13) -2524 (25) -3070 (22) -4044 (25) -3374 (56) 1368 (22) 1388 (32) 1320 (21) 2624 (47) 1024 (23) 381 (32) -463 (36) -1196 (43)
atom
Numbering schemes of phenyl rings and cations:
e,,, ,&d ,;( B r ( ' , a s ) q
5
6
,
li
''
,a
J,
2, a Q a
cm
-na z 0
+ a W LL
n
0
088
ln
m
a -800
-400
-2CC
000
200
400
80C
VELOCITY (MMISECI
Figure 5. Mossbauer spectra of polycrystalline (E4N),[Fe4S4(S-pC6H4Br)4]at 4.2 Kin external magnetic fields applied parallel to the y-ray direction: (a) H, = 60 kOe; (b) H, = 80 kOe.
i d l y delocalized with all four Fe atoms described by the mean oxidation state Fe2.5+.This situation approaches the limiting case of (Et4N)2[Fe4S4(SCH2Ph)4] ,26,27 Strict equivalence of all Fe sites was observed from 1.5 K to room temperature (6 = 0.34 mm/s; hE, = 1.25 mm/s at 1.5-77 K), with a relatively narrow line width I’ = 0.25 mm/s at 77 K. (b) (Et4N)3[Fe4S4(S-p-C6H4Br)4]. Zero-field spectra of the reduced cluster at 4.2 K in the solid state and in acetonitrile solution are presented in Figure 4. The solid-state spectrum arises from superposition of two doublets of roughly equal intensities, similar isomer shifts, and quite different quadrupole splittings. The appearance of two doublets in the solid-state spectrum is consistent with cluster C2 crystallographic symmetry, which affords two types of Fe sites, Fe(1,l’) and Fe(2,2’). Isomer shifts of the two sites (6, = 0.43 mm/s, 82 = 0.48 mm/s) fall within the range 0.43-0.52 mm/s observed for other [Fe4S4(SR),l3This cluster is also electronically delocalized, with the 6 values corresponding to the mean oxidation state Fe2.25+. The line widths of the doublets are comparable with those of other reduced clusters at 4.2 However, the difference in quadrupole splittings, 1.13 mm/s, is 0.2-1.1 mm/s larger than for those clusters. This, rather than intrinsically narrower absorption lines,28 accounts for the spectrum being by far better resolved than that of any other reduced cluster yet o b s e r ~ e d . ~ , ~ Because of improved spectral resolution the electronic nature of [Fe4S4(S-p-C6H4Br)4]3-has been examined, by use of magnetically perturbed spectra, in somewhat more detail than for other reduced cluster^.^^^ The application of an external magnetic field induces magnetic hyperfine structure, resulting in the spectra in Figure 5 . The complexity and broadening of these spectra arise primarily from unequal hyperfine coupling constants at the Fe sites, different orientations of the principal components of the electric field gradients (EFG) in the powder sample relative to the applied field, and a distribution of internal hyperfine fields owing to the orientation dependence of the hyperfine interaction. The two pairs of outer lines that flank the central unresolved absorption feature move in opposite directions as the applied field is increased from 60 to 80 kOe. This behavior is consistent with antiferromagnetic K.798
60
-40
-20
00
20
40
60
VELOCITY (MM/SEC)
Figure 6. Mossbauer spectra of polycrystalline (Et4N),[Fe4S4(S-pC6H4Br)4]at 4.2 K in an external magnetic field of 60 kOe applied parallel to the y-ray direction: (a) simulated spectrum of site 1 with 6, = 0.43 mm/s, AEQ,= 0.94 mm/s, rl = 0.62 mm/s, = -52 kOe; (b) simulated spectrum of site 2 with 62 = 0.48 mm/s, AEg, = -2.07 mm/s, = 1.12 mm/s, Hcff(2) = 102 kOe; (c) superposition of spectra a and b in a 1:l intensity ratio; (d) experimental spectrum. For clarity spectra a and b are not drawn to the scale of spectrum
C.
L
25
30
35
40
H IhG)
Figure 7. Solid-state X-band EPR spectrum of (Et4N),[Fe4S4(S-pC6H4Br)4]at -7 K.
spin coupling, which is a general property of reduced clusters and has been theoretically analyzed re~ently.~’The overall magnitude of the splittings at the two magnetic subsites is roughly comparable to those in the less well-resolved spectra of several [Fe4S4(SR),13-species (R = Ph, o-tolyl) in the solid statee7 The splittings are also similar to those observed in reduced Bacillus stearothermophilus f e r r e d ~ x i n . ~ ~ To obtain estimates of the effective fields (ITeff) at the Fe sites, the magnetically perturbed spectra were simulated by superposition of two sites of equal intensity corresponding to those observed at 4.2 K in the absence of an applied field. In Figure 6 the experimental spectrum at 60 kOe is presented together with simulated spectra. The latter were obtained by averaging over a random orientation of the molecular axes at the Fe sites relative to the applied field direction, as required for a powder sample. The number of parameters was minimized by considering only the case of an axially symmetric EFG (asymmetry parameter 7) = 0). Parameters used in the simulation of the spectra of sites 1 and 2 are specified in Figure
(27) Holm, R. H.; Averill, B. A.; Herskovitz, T.; Frankel, R. B.; Gray, H. B.; Siiman, 0.; Grunthaner, F. J. J . Am. Chem. SOC.1974, 96, 2644. (28) Site 1, with the marginally larger line width, is associated with the site
of the larger hyperfine field observed in the spectrum of this compound
in an applied magnetic field. This observation (vide infra) suggests that the lines are broadened by incipient magnetic hyperfine interactions.
(29) Papaefthymiou, G. C.; Laskowski, E. J.; Frota-PessBa, S.; Holm, R.H. Inorg. Chem. 1982, 21, 1723. (30) Middleton, P.; Dickson, D.P. E.; Johnson, C.E.; Rush, J. D. Eur. J. Biochem. 1978,88, 135.
1556 Inorganic Chemistry, Vol. 22, No. 10, 1983
Stephan et al.
6. The -6 and AEQvalues are taken from the zero-field spectra containing the only structurally characterized tetragonal at 4.2 K. The line widths of site 2, however, are considerably cluster, are distinguishable. broadened (from 0.56 to 1.12 mm/s). This is due to the fact The Mossbauer spectra of [ Fe4S4(S-p-C6H4Br)4]3in acetonitrile, DMF, and N,N'-dimethylacetamide solutions at that site 2 has the larger quadrupole splitting, and line 4.2-77 K are essentially identical. The spectra in Figure 4 broadening is expected to be introduced by the spatial averreveal a striking difference between the solid and solution aging of a random distribution of the EFG axes relative to the states. In the latter, two quadrupole doublets are evident but applied field direction. However, the large extent of broadare unresolved, a situation due in part to the smaller difference ening observed indicates that in addition there exists a disin AEQ values of the two subsites in solution. The solution tribution of effective hyperfine fields at that site. For simplicity spectra are indistinguishable from those of all other [Fe,S4this distribution was approximated by a single average value (SR),] 3- clusters previously e ~ a m i n e d . ~This . ~ situation can of Heffand a broadened line width. From the differential be appreciated by comparison of spectral parameters in Table broadening of the outermost absorption lines of the individual V, which also demonstrates the near-identity of these spectra subcomponents, the sign of the principal component of each to that of solid (Et3MeN),[Fe,S4(SPh),]. In acetonitrile soEFG was determined. At site 2 Heffis positive (parallel to lution at -7 K the EPR spectrum reduces to a simple axial the applied field direction), and at site 1 it is negative. On case, with g,,= 2.02 and g, = 1.92. This spectrum is the same the assumption that the electronic spins of the magnetic sites as that observed for all other [Fe4S4(SR),13-clusters in this are aligned with or opposite to the direction of the applied field, solvent. the hyperfine fields are estimated as Hhf(])zz -1 12 kOe and These results show that the structure of [Fe4S,(S-pHhf(2)zz 42 kOe. A similar analysis of the 80-kOe spectrum C6H4Br),13-in the solid and solution phases differs and that gave Ifhf([) zz -125 kOe and Hhf(2)= 45 kOe. The existence in solution the cluster approaches or adopts an elongated Dzd of sites with different hyperfine fields, together with associated core configuration closely similar to that of [Fe4S4(SPh),13Mossbauer spectral properties and antiferromagnetic interin a crystal of its Et3MeN+ salt.' Consequently, this work actions among these sites,29is consistent with the spin-coupled provides the second case of a structurally defined nontetragonal model of the electronic structure of the [4Fe-4S]+ oxidation cluster undergoing a core structural change to a tetragonal level.30 form in solution. Including cases of known and unknown Comparative Solid-State/Solution Properties. We have solid-state structures, [Fe4S4(S-p-C6H4Br)4]3-is the sixth previously classified [Fe4S4(SR),J3-clusters in the solid state example overall. Thus, the generality of the unperturbed core as having tetragonal or nontetragonal core structures on the structural alteration I + I1 (Figure 1) attendant to the basis of their electronic properties,s including those from electron-transfer process (2) is further supported. Mossbauer and EPR spectra. This classification is not inAs observed earlier, the compressed tetragonal core structure tended as literal, for a continuum of core structures ranging I is a consistent feature of all structurally defined synthetic from varying degrees of rhombicity to the tetragonal limit is clusters with the [4Fe-4SI2+ oxidation The same possible. Nonetheless, the collective properties in conjunction type of distortion is apparent for four [4Fe-4SI2+ protein with established tetragonal and nontetragonal structures c l u ~ t e r s , the ~ ~ most - ~ ~ recent example being the site in Azo(Figure 1) have led to a self-consistent partitioning of the bacter vinelandii ferredoxin I.35 A comparison of the structures clusters into the two categories. An important qualification, of analogue and protein clusters is available e l ~ e w h e r e . ~ evident in our earlier presentation^,^** is reemphasized. Crystal Electronic structural calculations for a Td core geometry of structure determinations of all reduced cluster compounds have this oxidation level by Aizman and Case36yield a MO energy been performed at ambient temperature. Magnetic and scheme in which the highest occupied orbital is filled. No static spectroscopic features correlated with tetragonal or nontetJahn-Teller effect is expected, and the factors responsible for ragonal [4Fe-4S]+ core structures have been measured at 577 the DZdgeometry remain unclear. If the same scheme holds K and frequently at 4.2 K. Clearly, retention of the essential for the [4Fe4S]+ core in Td symmetry, the odd electron would core structure at these lower temperatures is required for a occupy a t, or t2 orbital and Jahn-Teller distortion to a lower valid property/structure correlation. symmetry is possible. The three reduced clusters of known Mossbauer spectral criteria at 4.2 K for the two core structure are distorted (Table IV). Their nonuniform core structural categories are the following:s tetragonal-two structures presumably originate in specific crystalline-state overlapping quadrupole doublets with -6, AEQ = 0.50-0.52, effects. In none of these cases have we been able to discern 1.9-2.0 and 0.43-0.45, 1.1 mm/s; nontetragonal--6, AEQ clearly, from crystal-packing diagrams or from interion dis= 0.43-0.48, -0.9-1.4 mm/s. The spectral parameters of tance calculations, interactions that might exert a perturbing [Fe4S4(S-p-C6H4Br),13-in the solid state more closely apinfluence on core structures. The existence of such interactions proach those of the tetragonal category, a matter illustrated is well demonstrated by the essential degeneracy of the by comparison with the data for (Et3MeN)3[Fe4S4(SPh)4]in magnetic and spectroscopic properties of reduced clusters in Table V. EPR spectra at X-band have also been found to frozen solutions. A detailed description of the structural incorrelate with structures:s tetragonal-nearly isotropic or terconversion I + 11, its effect on electron self-exchange rates axial, absorption at 3.0-3.8 kG; nontetragonal-complex, of synthetic clusters, and the possible consequences for protein absorption extending over ca. 0.5-7 kG. Recent EPR lineredox properties by constraints imposed on this interconversion shape simulation^,^' together with magnetic susceptibility are given e I s e ~ h e r e . ~ ~ ~ - ~ * ' ~ indicate that spectra of the latter type can arise Acknowledgment. This research was supported by National wholly or partially from clusters with S > The spectrum Institutes of Health Grant GM 28856 at Harvard University of (Et4N),[Fe4S,(S-p-C6H4Br),]shown in Figure 7 falls nearer and by the National Science Foundation at the Francis Bitter to those in the tetragonal category. Given the nontetragonal structure of this cluster, these observations extend the range of Mossbauer and EPR properties associated with this (32) Adman, E. T.; Sieker, L. C.; Jensen, L. H. Acta Crystallogr., Sect. A 1975, A31, 534. structural category. However, the Miissbauer and EPR spectra S . T.; Alden, R. A.; Carter, C. W., Jr.; Kraut, J. J . Biol. Chem. of this compound and those of (Et3MeN)3[Fe4S4(SPh)4J,7(33) Freer, 1975, 250, 46.
-
-
(31) Collison, D.; Mabbs, F. E. J . Chem. SOC.,Dalton Trans. 1982, 1565.
(34) Carter, C. W., Jr. J . Biol. Chew. 1977, 252, 7802. (35) Ghosh, D.; O'Donnell, S.; Furey, W., Jr.; Robbins, A . H.; Stout, C. D. J . Mol. Biol. 1982, 158, 7 3 . (36) Aizman, A.; Case, D. A. J . Am. Chem. SOC.1982, 104, 3269.
Inorg. Chem. 1983, 22, 1557-1559
1557
Supplementary Material Available: Tables S-I-S-IV, showing anisotropic temperature factors, hydrogen atom coordinates and isotropic temperature factors, best-weighted least-squares planes, and mental assistance. observed and calculated structure factors, and a stereoview of Registry No. 1, 85269-29-0; (Et4N)2[Fe4S4(S-p-C6H4Br)4],(Et4N)3[Fe4S4(S-p-C6H4Br)4] (1 5 pages). Ordering information is given on any current masthead page. 85269-27-8.
N a t i o n a l Magnet Laboratory. We thank J. M. B e r g for structural calculations and Dr. P. K. Mascharak for experi-
Notes Contribution from the Department of Chemistry, Helsinki University of Technology, SF-02150 Espoo 15, Finland
Table I. Data for the X-ray Diffraction Study of U02S0,~2CO(NH2),
Uranyl(V1) Compounds. 3.' Crystal Structures of Two Forms of Bis(urea)dioxouranium(VI) SulfateZ Jukka Toivonen* and Lauri Niinisto
Received September 27, 1982
Uranyl sulfate forms several crystalline hydrates of the general f o r m u l a U0,S04.nH20. Detailed X-ray crystallographic studies of U02S04-21/2H203 and UOZSO4.31/2Hz0495 have been carried out earlier. The structures consist of infinite double chains formed by alternating U 0 7 and SO4polyhedra. In aqueous solution urea molecules are capable of replacing water molecules in the coordination sphere of the uranyl ion and complexes of the type U0,S04.nCO(NH2), may be precipitated by evaporation. The replacement of water by urea leads to different networks as seen in the structures of the tris(urea)6 and t e t r a k i ~ ( u r e a ) ~complexes. The former structure consists of single chains, w h e r e a s the latter is monomeric. T h e X-ray crystallographic investigation carried out in this w o r k has revealed that t h e bis(urea) complex forms two structurally related polymorphs. The crystal structures of the two polymorphs are described below. Experimental Section Crystals of both polymorphs of U02S04-2CO(NH2)2were obtained upon evaporation of an aqueous solution containing uranyl sulfate and urea in the molar ratio of 1:2 a t room temperature. Chemical analysis showed that the crystals have the same composition. However, the polymorphs differed in color. The greenish and yellowish crystals were distinguished by the prefixes a and 6, respectively. Fast evaporation yielded predominantly the /3 form. Preliminary DSC investigation revealed no polymorphic transitions for a and /3 forms. The crystal data given in Table I are in agreement with the unit cell values for the /3 form given by Durski et a1.* and with those reported recently by Serezhkin et al.;9 the latter authors have also prepared the a form. The details of the intensity data collection with a Syntex P21 single-crystal diffractometer are included in Table I. Empirical For part 2 of the series see: Niinistb, L.; Toivonen, J.; Valkonen, J. Acta Chem. Scand., Ser. A 1979, A33, 621. Presented in part at the 10th Scandinavian Meeting on Structural Chemistry, Helsinki, Jan 1981; see: "Book of Abstracts"; Association of Finnish Chemical Societies: Helsinki, Finland, 1981; p 76. Brandenburg, N. P.; Loopstra, B. 0. Cryst. Srruct. Commun. 1973, 2, 243. F'utten, N.; Loopstra, B. Cryst. Struct. Commun. 1974, 3, 377. Zalkin, A.; Ruben, H.; Templeton, D. Inorg. Chem. 1978, 17, 3701. Ruben, H.; Spencer, B.; Templeton, D.; Zalkin, A. Inorg. Chem. 1980, 19,776. See also: Serezhkin, V. N.; Soldatkina, M. A.; Trunov, V. K. Radiokhimiya 1981, 23, 678. Serezhkin, V. N.; Soldatkina, M. A.; Trunov, V. K. Koord. Khim. 1981, 7, 1880. Durski, 2.; WojtaS, H.; Boniuk, H.; Nowaczek, H. Acta Phys. Pol. A 1978, A53, 67 1. Soldatkina, M. A.; Serezhkin, V. N.; Serezhkina, L. B. Zh. Neorg. Khim. 1981, 26, 3029.
0020-1669/83/ 1322-1557$01 S O / O
cryst syst space group a, A b, a c,
a
P , deg
v,A 3 Z cryst size, mm fw p(calcd), g cm-3 p(obsd): g cm-' p(Mo Ka), cm-' cell const determn
(A) Crystal Data monoclinic p2, IC 6.856 (1) 12.760 (2) 11.571 (1) 91.73 (1) 1011.80 4 0.15 X 0.15 X 0.3 486.23 3.191 3.14 157.9 25 reflctns, 35 < 2s < 40°b
monoclinic p2, IC 6.747 (1) 14.110 (3) 11.348 (1) 106.53 (1) 1035.62 4 0.15 X 0.15 X0.25 486.23 3.118 3.08 154.3 25 reflctns, 35 < 2s < 40°b
(B) . , Measurement of Intensitv Data radiation Mo Ka, graphite monochromator reflections measd +h, +k,Tl 28 range, deg 5-60 s 12s scan type scan speed, deg min-' 2.0 scan range, deg (26'(Mo Ka) - 1)-(26(Mo Kor) + 1) bkgd measmt at beginning and end of the 20 range, each for half of the total scan time no. of reflctns measd 3311 3376std reflctns 111,202 141, 202 range of transmission 0.3 6-0.6 6 0.35-0.60 factors no. of unique data with 2554 2587 I > 30(I) extinction coeff g 0.00131 (4) 0.00129 (3) R = ,Z llFol - IF,II/C IFo] 0.060 0.069 Measured by flotation.
h(Mo Ka) = 0.7107.
absorption corrections were made from +-scan data, after which Lorentz and polarization corrections were applied to both sets of data. The structures were solved by the heavy-atom method and refined by least-squares techniques using anisotropic temperature factors for all non-hydrogen atoms. In the final stages of the least-squares refinement the calculated positional parameters for all hydrogen atoms were introduced but not refined (fixed isotropic temperature factor U = 0.10 A2). The extinction correction was applied for both structures. Scattering factors for the non-hydrogen atoms were according to Cromer and Mann,'' and those for hydrogen were according to Stewart et al." Anomalous dispersion correction was applied for uranium.I2 Calculations were carried out with the X-RAY 76 program system.I3 The final positional parameters are given in Table 11. Thermal parameters, hydrogen atom coordinates, and observed and calculated structure factors are available as supplementary material.
(x) Cgmer, D.; Mann, J. Acta Crystallogr.,Sect. A A24, 321. (11) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J . Chem. Phys. 1968,
-.
1965,
4 2 3175 - - -.
(12) Cromer, D. T.; Liberman, D. J . Chem. Phys. 1970, 53, 1981. (13) Stewart, J. M., Ed. 'The X-Ray system, Version of 1976" Technical Report TR-446; Computer Science Center, University of Maryland: College Park, MD, 1976.
0 1983 American Chemical Society
